Follow the Ubuntu instructions.

Create 2 consecutive partitions on your hard drive

• partition 1 : type = fat32, name = BATOCERA
• partition 2 : type = ext4. It must be created just after the BATOCERA partition.
• download boot.tar.xz and unzip it on the BATOCERA partition.

$ sudo wget https://batocera.org/upgrades/grub/15_batocera -O /etc/grub.d/15_batocera
$ sudo chmod a+x /etc/grub.d/15_batocera
$ sudo update-grub
...
Image batocera found on 9A01-4A3B
...